1. Field of the Invention
The present invention relates to a three-dimensional object image drawing apparatus and a three-dimensional object image drawing method for drawing a three-dimensional object image on a display screen, and more particularly to a three-dimensional object image drawing apparatus and method for rotating or moving along a line a three-dimensional object image displayed on a screen.
In recent years, CAD systems using Engineering Work Station (EWS) have been developed and have been used in various design work. In such a CAD system, means for communicating between a person and a system, namely, a man-machine interface, is regarded as important for handling images. Therefore, a man-machine interface that provides easier operation is desired.
To this end, instead of a conventional keyboard, a pointing device such as, for example, a mouse has been developed as a means for effecting communication with a system. By using the pointing device, figures can be easily drawn or edited.
In such a CAD system, frequently there are cases in which an image of a three-dimensional object having a size larger than the size of the screen in the system must be displayed. In this case, the image is displayed by scaling down the figure or by partially displaying the figure. For example, when the whole figure is to be seen, the display is scaled down, and when a part of the image is to be displayed, a part of the image of the three-dimensional object is selected and displayed.
In addition, in a CAD system, it is necessary for design purposes to display a three-dimensional object when it is viewed from an arbitrary direction.
In a conventional CAD system, to partially display a desired three-dimensional object image, a displayed figure is modified by moving it along a line in such a way as to designate and instruct, by means of a pointing device such as a mouse, for example, a predetermined point on the three-dimensional object image, currently being displayed. The designated point is moved to the center of the screen, resulting in a partial display of the three-dimensional object image. This movement of an image is hereinafter referred to as a parallel movement. To designate and instruct as above is referred to as "pan hit".
Also, to rotate a three-dimensional object image by changing the view direction, it is necessary to know the angle of rotation with respect to each axis and to designate the desired angle of rotation with respect to the axis.
When a parallel movement is to be effected by the conventional CAD system, however, the three-dimensional object image is moved along a line and displayed only in such a way that the pan hit point, i.e., the designated and instructed point, is moved to the center of the screen. Therefore, when a predetermined range of the figure is to be displayed, an operator cannot easily recognize the pan hitting position, i.e., the designating and instructing position, to display the desired range so that, in order to display the desired figure on the screen, pan hittings, i.e., designations and instructions, must be repeated many times. Further, by repeating the pan hits, if the operator misses the current position, the picture image must be compressed one time and a cursor must be moved to a desired position, and then the image must be enlarged again to pan hit it. Therefore, in the conventional CAD system, there is a problem in that to display a desired image by means of a parallel movement is very complex and inefficient.
Also, in the conventional CAD system, when a three-dimensional object is to be rotated by changing the direction of view or view direction, there are problems in that an operator must recognize and designate the angle of rotation with respect to each axis in accordance with the change of the view direction, making the operation complex. Moreover, the image when viewed from the desired view direction cannot always be displayed at one time but may require many separate operations.